Multiple labels will mean multiple copies of a message will exist across those folders

The shortcut key for assigning a Gmail Label is "L"

You can change the color associated with a label by right clicking on the label in the folder pane and choosing Label Color from the context menu. Gmail labels must contain at least one color, and there is no current way to change the color for multiple labels at once.

Notes:

In order to keep labels in sync, Postbox will check all Gmail folders for new messages. To prevent Postbox from checking all Gmail folders for new messages:

In Postbox you will find within the "[Gmail]" folder "All Mail" and "Starred." All Mail contains a copy of every message in your Gmail account, so this folder can be quite large.

If a message in Gmail has a label, then a copy of that message is created in Postbox. For example, a message in Gmail's Inbox will be found within the Inbox and All Mail folders in Postbox. If a message in Gmail's Inbox contains the label "Holiday," then a copy of that message will be found the Inbox, Holiday, and All Mail folders in Postbox.

Postbox Tips for Better Performance:

If you do not wish to view or download messages within All Mail, you can unsubscribe from this folder, or limit the number of messages that display in the All Mail Folder. Please see this article for details.

Getting Folders Correctly Mapped

Trash Folder

In Preferences or Options > Accounts > YourGmailAccount > Account Settings, towards the bottom of this panel is a setting for deleting a message.

Make sure that "Move it to this folder" is selected.

Note: You cannot select a custom folder, as Gmail doesn't perform well when other folders are specified for Trash. You can learn more about changing the delete behavior for Gmail account here.

Sent Folder

In Preferences or Options > Accounts > YourGmailAccount > Copies & Folders, make sure that "Place a copy in…" is NOT checked. Gmail will automatically add sent messages to the sent folder on the server side.

Double-click pb.gmail.label_delete.useAllMail to change the value from true to false.

This change does not impact special labels/folders such as the Inbox, Archive or Sent. Within a special folder like the Inbox, deleting moves the messages to Gmail/Trash, causing it to be removed from all other labels/folders.

For more detailed information on delete behavior, please see this article.

Working with Spam

When you mark a message as "Junk" in Postbox, it is moved to Gmail's "Spam" folder. However, emptying the Spam folder in Postbox does not empty the Spam folder in Gmail. Gmail on the web will automatically delete messages that have been in the Spam folder for more than 30 days.

Gmail Starred Mail and Postbox Reminders

Email messages contain a hidden keyword called a "flag," and different email clients have different names and symbols for flagged messages. In Gmail, they are called "Starred" messages, in Apple Mail they are "Flagged" messages, and in Outlook they are called "To-Dos." Postbox also associates flagged messages as Reminders.

In short, Starred Messages in Gmail = Reminders in Postbox.

Control IMAP Folder Size Limits

Gmail (and Google Apps) users have the opportunity to put a limit on how many messages will sync per folder. To use this feature, go to Mail Settings > Forwarding and POP/IMAP and find the Folder Size Limits option at the bottom. Choices range from 1,000 to 10,000.